The present invention relates to an alignment technique for exposure in fabrication of a semiconductor device, and more particularly to a projection alignment method and apparatus suitable for alignment in the case where uneven application of a resist film takes place.
The conventional projection alignment method and apparatus have been disclosed by, for example, JP-A-5541739 (corresponding to U.S. Pat. No. 4,362,389). A main part of the apparatus is shown in FIG. 13. A wafer 3 is exposed to a circuit pattern 120 delineated on a mask (or reticle) 1 through a reduction projection lens 2 which reduces the circuit pattern to one several. The exposure is made one chip by one chip or plural chips by plural chips.
Generally, a semiconductor device is formed through successive superimposed printing of several kinds of circuit pattern. Upon exposure, therefore, it is required to accurately align the circuit pattern 120 on the mask 1 and a circuit pattern 121 already formed on the wafer 3 with each other. This is carried out by detecting a mask alignment pattern 126 on the mask 1 and a wafer alignment pattern 14 on the wafer 3 through optical systems 124 and 125 for alignment and feeding back the amount of positional deviation between both the patterns to a wafer stage 123 as the amount of alignment. FIG. 14 shows the wafer alignment pattern 14 in an enlarged form thereof. The wafer alignment pattern 14 is made of a concavely or convexly stepped pattern (or a pattern involving a level difference therein) and is illuminated with illumination light 122 for alignment in a state in which a resist film 53 is applied on the pattern 14. Reference numeral 52 designates an underlying substrate.
Since the alignment illumination light 122 used in the conventional projection alignment method and apparatus is monochromatic light having a wavelength which is substantially or nearly equal to the wavelength of light used for exposure, a detection signal of the wafer alignment pattern 14 is affected by multiple interference which may generate in the resist film, depending on the thickness of the resist film. Therefore, for example, in the case where the thickness distribution of the resist film in the vicinity of the wafer alignment pattern 14 is symmetrical as shown in FIG. 15A, the detection signal 127 becomes symmetrical as shown in FIG. 15B. But, in the case where the resist film thickness distribution is asymmetrical as shown in FIG. 16A, the detection signal 127 becomes asymmetrical as shown in FIG. 16B. Accordingly, in an alignment method in which the center position of the wafer alignment pattern 14 is determined by virtue of the symmetry of a waveform of the detection signal, the latter case would result in the degradation of detection precision and hence the degradation of alignment precision.